1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image, or a toner for forming a toner image in the image forming process by toner jet method, and to a process for producing the same.
2. Description of the Prior Art
In electrophotography, a number of methods are known as disclosed in U.S. Pat. No. 2,297,691 and Japanese Patent Publications No. 42-23910 and No. 43-24748. In general, copies or printed materials are obtained by forming an electrostatic latent image utilizing a photoconductive material according to various means on a photosensitive member, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper if necessary by direct or indirect means, followed by fixation with heat, pressure, heat-and-pressure, or solvent vapor. The toner which is not transferred and left on the photosensitive member is removed by various means if necessary, and then the above process is repeated.
Such toners as mentioned above are powders which generally comprise a binder resin and a colorant as main components, and further contain a charge control agent and a fixing assistant and the like. In general, the particle size thereof is in a range from a few microns to about 20 to 30 microns. Such toner has been typically produced by so called pulverization method, in which a colorant such as a dye, a pigment or a magnetic substance is mixed in a thermoplastic resin and melted, so that the colorant is homogeneously dispersed in the thermoplastic resin, then it is pulverized and classified.
In recent years, an image forming apparatus employing electrophotography has been widely used not only as a copying machine for office work to take copies of original texts, but also as a full color output machinery of high image quality, and a high-resolution output machinery connected with computers. In addition, as the computers have been used for general purposes, the printer has also been used as a personal printer for private use, therefore, there has been a demand for high-resolution and quality images in various environments.
Consequently, the requirement for toner performance has become higher and higher; because without the improvement of the toner performance, that allows the images to be of high quality in various environments, excellent image formation cannot be carried out.
One of the means to achieve such quality images is a method of reducing the particle size of the toner. Indeed, by reducing the particle size of the toner to several microns, the picture quality and the resolution have been improved.
However, for the toner which has been produced in the conventional pulverization method, it has been difficult to provide a particle having a diameter of not more than about 5 .mu.m, since when high impact force is applied to the particles to reduce the size thereof, the pulverized substances may be fused onto the pulverizing apparatus. Also, in the classification procedure, when the particle size of a toner is reduced, the cohesive force of the powder particle is increased, and it requires great effort to provide a toner having a sharp particle distribution. When the particle distribution of the toner is broad, it becomes difficult to control the charge quantity of the toner and problems such as scattering or spattering of the images or fogging on the images tend to be caused.
In order to improve the reduction in the particle size of the toner, and to have a sharper particle size distribution of the toner, a process to produce a toner by polymerization has been proposed. For example, Japanese Patent Publication No. 6-52432 and Japanese Patent Laid-Open No. 5-93002 disclose a method of producing particles of around 1-10 .mu.m, having a sharp particle size distribution. In a method disclosed in Japanese Patent Publication No. 6-58543 and Japanese Patent Publication No. 6-58544, particles for image formation having a sharp particle size distribution are produced, wherein, in addition to the sharp particle size distribution, the particles have stabilized charge characteristics and improved functionality since they are coated with a colorant or a conductive substance and a binder resin.
Though such particles having a sharp particle size distribution have excellent fluidity, they are formed by closest packing. Therefore, particularly when these particles are left in a high temperature environment, a problem of aggregation of the toner arises. When the particles are coated with the colorant or a conductive substance and the like in order to acquire the above-mentioned functionality, aggregation occurs more easily in the case of closest packing due to the unevenness in the microstructure on the toner surface. When a toner or a developing agent causes aggregation, charging failure tends to occur and that ultimately leads to a problem of inferior resolution of the developed image.
When a conductive substance is coated on toner particles, the toner particles have appropriate charges in the normal temperature and normal humidity environment. However, they cannot maintain the charges under high humidity conditions, and problems such as fogging occur due to the charging failure.
In Japanese Patent Laid-Open No. 10-232509, a toner for developing an electrostatic image has been proposed, in which the maximum glass transition point of a methanol soluble resin component which is extracted by a Soxhlet extractor employing methanol, and the maximum glass transition point of a tetrahydrofuran (THF) soluble resin component which is extracted by Soxhlet extractor employing tetrahydrofuran following the extraction with methanol, satisfy a specific relation. The toner proposed in that publication allows good image formation under a high temperature condition, however, image formation under high humidity condition and image formation under low temperature and low humidity conditions are still in need of improvement.